a. Field of the Invention
This invention concerns the field of pressurized high frequency heating of plastic molded objects with the material placed in a hollow mold equipped with raised patterns corresponding to the designs to be obtained on the objects in question. It especially concerns improvements in the technique for molding in molds of silicone elastomers, as well as a device for injection molding adapted to high frequency heating press equipment.
b. Description of the Prior Art
Various molded objects, for example, the components of quarters and vamps for shoes, fancy leather goods or other items, are produced by filling the hollow parts of a mold constituting negatives of the design to be reproduced with a plastic material such as polyvinyl chloride in powder or liquid form. Then, after possible application on the mold of a material which will constitute the lining, the sandwich assembly is exposed to high frequency (HF) heating, under pressure, after which a cooling makes it possible to obtain an instanteous mold stripping of the formed plastic object. Such a technique, with its different variants, as well as the appropriate equipment, has already been described in a series of patents owned by the present assignee.
It is known that by submitting a dielectric material, for example, a macromolecular plastic, to current whose frequencies correspond to the molecular resonance of the material in question, one obtains a heating due to dielectric losses. For example, in this technique, one generally submits polyvinyl chloride powder (PVC) to frequencies of approximately 27 megacycles or more, under pressure, in molds of silicone elastomer, to obtain, in less than on minute, shaped objects, corresponding to the designs of the mold. In practice, the mold is built using a mixture of silicone with a metal powder, such as aluminum, in order to provide a better distribution of the high frequency field and to enable a better cooling of the molded PVC.
Various types of molds especially adapted to this technique have been proposed by the present assignee, particularly including molds in which the pure resin is more or less filled, in certain constituants, with metal powder or with an equivalent material (French Pat. No. 78.32827, 78.33917 and 79.06692), or molds which can receive at least a metal insert (French patent No. 80.03658).
For molding according to the technique reviewed above, the plastic material (PVC or other) is introduced in the form of powder or it may be poured in the form of liquid plastisol into the hollow parts of the silicone mold, with or without metal insert. Upon initial examination, it could not be expected that injection molding could be used because it would require the use of metal molds under very high pressures, often from 10 to 50 megapascals (MPa).